Coatings for hard and soft surfaces are important for the protection of the surface and can add to the aesthetic appeal to natural surfaces, such as wood. Concerns about emissions of volatile organic compounds (VOCs) along with stricter legislation on VOCs have led to an increased focus on the use of water based systems for coating applications. Water based systems offer the additional advantages as they are easier to use than solvent based coatings.
Polyurethane dispersions (PUDs) may be used for a wide range of commercial applications, such as coatings for textile fabrics, plastics, wood, glass fibers and metals. The production of polyurethane-urea dispersions is known, as exemplified in U.S. Pat. Nos. 4,237,264; 4,408,008; 5,569,706; U.S. Patent Publications 2004204559; 2005004367; GB Patent Publication 2386898 and WO Publication 2005023947.
When aromatic isocyanates are used in the formation of coatings, such coatings tend to yellow over time when exposed to sunlight or UV radiation. To give coatings with improved non-yellowing characteristics, aliphatic isocyanates are used. While giving improved non-yellowing characteristics, coatings based on aliphatic isocyanates generally have reduced hardness and reduced solvent resistance as compared to polyurethane coatings based on an aromatic isocyanate.
It as an object of the present invention to provide aqueous PUDs for surface coatings based on aliphatic isocyanates which are capable of forming a film at ambient temperature without the need for a coalescing solvent. It is a further object of the present invention to provide an aliphatic isocyanate based PUD which when applied to a surface gives a hard, solvent resistant coating.